Many wound dressings comprise a wound contacting portion and a microorganism-impermeable backing sheet that covers the wound contacting portion. The wound-contacting portion, which may comprise several layers, is typically absorbent and may be therapeutic. In use, the backing sheet holds the wound contacting portion of the dressing in contact with the wound, blocks the ingress of microorganisms to the wound, and also prevents leakage of wound exudates from the dressing. In certain embodiments, the backing sheet is substantially coterminous with the wound contacting portion. In other embodiments, the backing sheet is larger than the wound contacting portion, such that a margin having width 1 mm to 50 mm, suitably 5 mm to 20 mm, extends around the wound contacting portion to form a so-called island dressing.
Conventional polymers for forming the backing sheet include polyurethanes and poly alkoxyalkyl acrylates and methacrylates such as those disclosed in GB-A-1280631. Typically, the backing sheet comprises a continuous sheet of a high density blocked polyurethane foam that is predominantly closed-cell. Well known backing sheet materials include the polyurethane films available under the Registered Trade Mark ESTANE.
The backing sheet is frequently coated with a medically acceptable adhesive to bond the backing sheet to the wound contacting portion, and/or to the skin of a patient around the wound. For example, in typical embodiments the backing sheet is adhesive coated at least in a marginal region thereof. As previously described, the backing sheet suitably extends beyond the outer edges of the wound contacting sheet to provide an adhesive-coated margin around the wound contacting sheet for attachment of the dressing to skin around a wound. The adhesive material can be moisture vapour transmitting, for example it may be a hydrogel adhesive. However, the more commonly used pressure-sensitive adhesives are not moisture vapour transmitting. These adhesives are typically printed onto the backing sheet in a patterned (discontinuous) fashion to allow passage of water vapour through the adhesive layer. The step of printing the adhesive adds expense and complexity to the manufacturing process.
Existing adhesive-coated backing sheets are generally quite thick. This thickness is needed in order to give the sheet sufficient stiffness for convenient handling prior to application to the wound. The adhesive layer is also typically quite thick, for example 100 to 250 g/m2. Where a polyurethane foam backing sheet is used, the adhesive layer should be continuous in order to provide the necessary barrier to microorganisms. These thick materials are relatively expensive, and render the desired oxygen- and moisture-permeability difficult to achieve.
Accordingly, a need exists for adhesive backing sheets for use in wound dressings that combine the advantages of low cost, sufficient stiffness for easy handling, and high moisture vapour permeability.